1. Field of the Invention
The invention relates to equipment for arc welding, and more specifically, it deals with an apparatus for overhead submerged-arc welding.
2. Description of the Related Art
A large number of operations are performed in the manufacture of welded structures with welding of rotatable annular welds of hollow products with a restricted access to joints being welded from the interior. Such joints include annular joints of closed vessels, annular joints of pipelines, tanks, casings, assembly welds, shell plating seams of ship hulls, and longitudinal welds of large-area products which it is difficult to place to a position facilitating welding. Such joints also include joints between difficult-to-position webs, segments, three dimensional and planar sections and other members.
The overhead submerged-arc welding method is characterized by the fact that a consumable electrode and welding bath are turned at 180.degree. in comparison with the downhand submerged-arc welding. Flux and electrode are supplied from bottom up, i.e. as though towards a ceiling. The electrode is supplied through compacted flux.
This is why this welding method will be referred to hereinbelow as overhead submerged-arc welding.
This welding method is referred to as the overhead submerged-arc welding also because arc is in the body of metal.
So called overhead welds are produced as a result of such welding.
Overhead welds and sealing overhead welds. There may also be one-pass overhead welds and other overhead welds.
The penetration overhead welds are the welds which are first to be produced in welding a joint and which are located in the top part of sections being welded, on the opposite side of the joint with respect to the electrode supply. Further welding of the joint, i.e. producing further welds, can be carried out by any appropriate knowns method, the electrode being supplied on the same side as was the case in producing the penetration overhead weld, e.g. producing inner penetration welds of rotatable annular joints of vessels, tanks, joints between bottom sections of shell plating of ships, and other structures.
The overhead submerged-arc welding of penetration welds mainly allows welding inside vessels in producing rotatable annular welds to be eliminated, and welding can also be avoided in confined spaces in producing straight welds of structures with an access on the side opposite to the ceiling.
The sealing overhead welds are the welds which are first to be produced in welding a joint and which are located in the bottom part of sections being welded on the joint side in the vicinity to the electrode supply. Further welding is carried out by any appropriate known method, the electrode being supplied on the opposite side of the joint as compared with the overhead welding.
In practice, the penetration overhead welds are produced in welding annular and longitudinal joints of structures with a restricted access to joints being welded from the interior.
The sealing welds are produced in welding elongated longitudinal joints of difficult-to-position products such as plate structures made out of segments and other members.
The one-pass overhead welds are the welds produced in welding joints of a limited thickness located over the whole welded section. No further welding of the joint on either side is required.
Many problems arise in producing sealing and one-pass welds in forming the surface of the finished weld.
The metal in the welding bath formed during arcing by fusion of the metal being welded, electrode material and welding flux is held by the crust of partly melted flux and by forming means. Forming means may be of various configuration and size and may be, e.g. in the form of plates, backings, bars, sliders and other members.
Flux is positively pressed against the welding spot from bottom, and as flux is consumed, its stock is continually replenished. Flux may be supplied for forming the top part of the weld either on the bottom side through the gap between the edges of welded members or from top by any appropriate known method so as to form a filled flux layer. Special forming backings or flux holders may also be used.
Special problems arise in welding joints of large-size cylindrical or like products such as ship hulls and boiler units where especially high quality of welds is required and where the product should be rotated about its axis during welding, and also in welding large-size planar members which it is difficult to place to a position facilitating welding.
Known in the art is an apparatus for overhead submerged-arc welding (SU, A, 1348111), comprising a hopper containing flux pivotally mounted on a pivot pin and accommodating a bowl having its open part facing towards a work being welded. The bowl communicates with a flux supply pipe having inlet and outlet ports, and an auger provided in the pipe and having a drive for supply flux to the bowl and pressing it against the work. A welding nozzle for supplying a consumable electrode extends through the bowl. The apparatus has a pair of copying wheels, one wheel being provided on the front part of the hopper in the welding direction and the other being aligned with a forming means. The hoper supports a welding head. The hopper is mounted on a pivot pin. The same pivot pin supports the other copying wheel and the forming means.
The apparatus is provided with means for turning the hopper about its pivot pin and a means for pressing the second copying wheel and forming means against the work being welded.
The forming means is provided in the vicinity to the nozzle to be extend above the brim of the open part of the bowl.
The pivot pin supporting the hopper, forming means and second copying wheel is mounted on a mounting arm provided on a carriage having means for pressing the forming means with the copying member against the work being welded, which comprises a power actuator for moving the carriage towards the work being welded.
The prior art apparatus is so constructed as to allow both absolute values of flux pressure in the bowl and force with which the forming means is pressed against the work being welded and the ratio between them to be varied.
This apparatus allows high-quality overhead submerged arc welding to be carried out with the desired forming of weld on either side in a broad range of process capabilities and with various types of products being welded.
However, as the forming means and the second copying wheel in this apparatus are mounted on one and the same pivot pin, the forming means is pressed away from the work in case of a substantial convexity of the joint being welded on the underside of the work. This results in a substantial change in position of an axis of oscillations of the hopper and copying wheel with respect to the surface of the work being welded. This change in position of the axis of oscillations of the hopper results in material fluctuations of preset pressures of flux in the bowl and at different points where welding bath is formed (upstream the arc, in the arc zone and in the welding bath zone and at the point of solidification of the welding bath), and in a disruption of welding as a whole, impairing quality of the welded joint.
In cases of a substantial concavity of the joint being welded on the underside of the work, the forming means is separated therefrom so that a substantial surplus space is formed between the working face of the forming means and the work to disrupt welding.
In addition, in welding products with geometry and assembly errors of joints, the clearance between the bowl and work fluctuates in the zone between the two copying wheels which are in contact with the work so as to result in a change in flux volume between the bowl and work thus causing fluctuations of flux pressure in the bowl and impaired welding quality as a whole.
Also known in the art is an apparatus for overhead submerged-arc welding (DE,C, 3430394), comprising a suspended pivotally mounted hopper containing flux and accommodating a bowl having its open part facing towards a work being welded and communicating with a flux supply pipe having inlet and outlet ports and an auger provided in the pipe having a drive for supplying flux to the bowl and pressing it against the work. The pipe, auger and drive for supplying flux to the bowl and pressing it against the work form a driven auger feeder. A welding nozzle for supplying a consumable electrode extends through the bowl. A copying member is provided adjacent to the welding zone and is engageable with the work surface during welding. A forming means provided adjacent to the welding nozzle above the brim of the open part of the bowl is mounted on a suspension for oscillations in its own longitudinal and transverse planes. The hopper is pivotally mounted on a mounting arm for rotation about its pivot pin.
The pivot pin supporting the hopper is mounted on the mounting arm which is movable in the direction towards the work being welded.
The same mounting arm supports the forming means and the copying member mounted on pivot pins.
The support pivot pin of the forming means is in the form of a point-like abutment at the end of an arm of a double-arm suspension lever of the forming means. The fulcrum of the double-arm lever is mounted on a mounting arm, the other arm of the lever being connected to a power actuator pivotally attached to the mounting arm. The copying member is in the form of a copying wheel and is mounted on the pivot pin supporting the hopper.
In another embodiment of this apparatus, the pivot pin supporting the forming means comprises a point like abutment at the end of a mounting arm located adjacent to the welding nozzle.
In this embodiment of the apparatus the copying member is in the form of projections on the face of the forming means, the pivot pin supporting the hopper is located on the side of the forming means remote from the nozzle, and the hopper is provided with a means for moving it with respect to its support pivot pin.
In this apparatus the mounting arm is mounted on a carriage which is mounted together with its drive on a driven trolley for moving the whole apparatus in the welding direction.
This apparatus allows permanent contact between the forming means and copying member and the work being welded to be ensured during welding with various assembly errors and deviations from geometry of the joint being welded (e.g., misalignment of plates edges, convexities and concavities, undulations, clearances, and the like).
Therefore, upon any change in position of the forming means during welding caused by an admissible change in profile of the surface of the work being welded at a point of their contact, the eventual action of the forming means upon position of the hopper containing flux is eliminated.
This facility makes it possible to prevent undesirable oscillations of the hopper upon changes in profile of the surface of the work and to stabilize such welding parameters as thickness of a flux backing and flux pressure in the welding zone.
The flux backing is an area of compacted compressed flux layer which has a preset pressure distributed over the whole area of the flux backing and which is located between the top part of the bowl facing towards the work being welded and the surface of the work being welded to exert a local pressure upon the surface of the joint being welded in the welding zone.
This construction of the apparatus provides conditions for a smooth copying of the surface of the joint being welded by the forming means without jerks and shakes which is necessary for maintaining stable preset values of flux pressure acting upon the welding bath and upon zones in which the welding bath is formed along the joint being welded.
The construction of the prior art apparatus also provides conditions for a smooth copying surface of the joint being welded by the forming means and for adjusting position of the hopper with respect to the work being welded which is necessary for carrying out welding of products of different geometry.
In this apparatus, a change in position of the forming means during welding which occurs because of errors of geometry and assembly of the joint being welded does not cause a change in position of the pivot pin supporting the hopper with respect to the surface of the work being welded.
In welding with such an apparatus, owing to the creation and maintenance at a constant level of preset flux pressures at various points along the joint being welded (upstream the arc, in the zone of the arc and welding bath and in the zone downstream the welding bath and up to the formed weld), the possibility of automatic conduct of welding of overhead welds and production of high-quality welded joints is ensured.
This apparatus makes it possible to carry out welding with a desired formation of weld reinforcement on either side of the weld over a broad range of process capabilities with a wide range of welded products and with large errors of assembly of joints before welding (misalignment of plates edges, undulations, convexities and concavities, taper of bottom in welding annular joints, e.g. in boilers and railway tanks, and the like welded structures).
Investigations showed that the flux backing thickness (the space between the surface of the outlet port of the bowl and the surface of the joint being welded filled flux) should be kept constant during automatic overhead submerged-arc welding during the entire welding period. This is required to provide conditions during welding for the maintenance of constant preset flux pressures at various points along the joint being welded (upstream the arc, in the zone of arc and welding bath and in the zone of solidification of the weld) and for retaining the welding bath at the level of the joint being welded.
In the above described apparatus, flux which is supplied during welding by means of a non-controlled driven auger feeder moves along a close-loop circuit: supply of preset amount of flux through the supply pipe with an auger from the hopper to the bowl and spillage back of unfused flux that did not participate in welding from the bowl with subsequent supply thereof during the next cycle.
Flux is comminuted (flux grading composition changes) after each cycle of its movement so as to result in compaction of flux as shown by investigations, hence, in a lower throughput capacity of the auger feeder (reduction of the amount of flux supplied by the auger to the bowl per unit of time), hence, in a decrease in the flux backing thickness.
However, in welding elongated joints at a constant speed, i.e. in applications where multiple cycles of passage of flux through the auger feeder occur with multiple comminution of the flux, the prior art apparatus cannot maintain the preset flux backing thickness at a constant level. The flux backing thickness decreases because of comminution of flux and associated reduction of its particle size as the weld length, hence number of recycles of flux increases. In addition, in welding with such an apparatus of long welds with a varying welding speed, the flux backing thickness varies upon variation of the welding speed.
Such changes in the flux backing thickness result in fluctuations of preset flux pressures at various points along the joint being welded during welding, impair welding quality and disrupt the process as a whole.
In such cases, welding should be interrupted, and the apparatus stopped. The comminuted (pulverulent) flux should be replaced with fresh flux having an optimum grading composition. After replacement of the flux, the end crater of the weld should be repaired, and only after that can welding of the long joint continue. Quality of welding of such joints is impaired because of stoppages, repaired portions and other defects.
In addition, this apparatus does not allow a preset flux backing thickness to be set up or varied if necessary in welding long joints (e.g. in welding structures with different thicknesses of plates along one and the same joint, e.g. in the manufacture of railway tanks and other products), nor does it allow flux to be rapidly replaced by another grade or type of flux (fine-grained, ceramic, and the like) during welding at a constant or varying welding speed and with the employment of an arc auger drive (constant speed motors which are generally used under heavy-duty industrial operating conditions).
On the other hand, when a d-c drive is used for driving auger in this apparatus (with a speed control), the flux backing height can be controlled during welding of long joints. This control in the prior art apparatus can, however, be only affected by way of visual monitoring with measurement of the actual thickness of the flux backing during welding at regular intervals, comparison thereof with a preset value and manual variation of the auger speed, e.g. by means of potentiometers in the d-c motor control circuit in the auger rotation drive so as to maintain the preset thickness of the flux backing. This adjustment and maintenance of the flux backing thickness cannot ensure a guaranteed quality in producing overhead welds since operation depends on skill and experience of the operator and calls for a permanent involvement of the operator in welding process.
Overhead submerged-arc welding process becomes more difficult, and quality of welded joints is impaired in all such cases.
Therefore, in order to enhance and facilitate welding process in welding at a constant and varying speed in carrying out the overhead submerged-arc welding and to ensure the production of high-quality welds of a constant quality along the entire joint being welded, it is necessary to carry out automatically an increase in flux supply to the welding zone (to the bowl) upon every close-loop cycle of the flux movement, and flux supply to the bowl should also be changed upon every change in welding speed so as to ensure a constant flux supply rate to the welding zone.
This operation with a constant flux supply rate to the bowl ensures the maintenance of a constant preset flux backing thickness necessary for a high-quality overhead welding in welding long joints at a constant or varying speed.